Valve has historically been good about this, but the point is that the past doesn't guarantee the future. You cannot trust them to keep a thing for any amount of time without something legally binding.

Steam also can't do anything to protect you from games that require servers to play if those servers are shut down. Case in point, "Concord". They can offer direct refunds (I know they did that with The Day Before), but then doing that is a PR move that you can't rely on.